The control of wafer flatness is essential in particle-beam, e.g., electron-beam, lithography. While undergoing lithographic processing, semiconductor wafers may be secured in chucks by mechanical clamps, gas pressure differences created by vacuum pumping, or electrostatic attraction. Because vacuum chucks are not useful in the evacuated environment of a particle-beam system, mechanical clamping is most widely used. However, mechanical clamps are unable to secure the central portion of the wafer, and bowing of the wafer in excess, for example, of 10 .mu.m at the center of the wafer has been observed when the periphery of the wafer is held by mechanical clamps. This is undesirable because 10 .mu.m exceeds the depth of focus of at least some particle-beam lithographic tools.
Electrostatic chucks, by contrast, are capable of securing the entire lower surface of the wafer by Coulombic force. As a consequence, the flatness of the wafer is improved relative to mechanical clamps. In a typical electrostatic chuck, the wafer effectively forms one plate of a parallel-plate capacitor. The other plate is formed by the upper surface of a conductive body, here called the charge plate. (By upper surface is meant the surface facing the approaching actinic radiation. By actinic radiation is here meant particle beams, e.g. electron beams, as well as electromagnetic radiation capable of causing chemical changes.) The upper surface of the charge plate is made extremely flat and is coated with a thin dielectric layer. The dielectric layer separates the charge plate from the wafer when the capacitor is assembled. Means are provided for applying a first potential, e.g., ground potential, to the wafer, and for applying a second potential different from the first to the charge plate. As a result, the capacitor is charged, and the wafer is electrostatically attracted to the charge plate and flattened against the dielectric layer.
It is advantageous to detachably mount the electrostatic chuck in a cassette, as described, for example, in U.S. Pat. No. 4,412,133, issued to W. A. Eckes, et al., on Oct. 25, 1983. The use of a cassette makes it possible to assemble the wafer onto the chuck outside of the lithographic system.
The cassette described by Eckes, et al., includes a parallelepipedal body thick enough to receive the charge plate and wafer within a central opening passing through the body. When the wafer and the charge plate beneath it are assembled in the opening, retaining means stop the vertical advancement of the wafer through the opening at a point such that the upper surface of the wafer is vertically aligned with a plane that coincides with the focal plane of the lithographic tool when the cassette is aligned within the lithographic system. Holding means press against the lower surface of the charge plate and keep the assembly intact. The cassette is loaded outside of the lithographic system by assembling the charge plate and wafer within the central opening. After the cassette is loaded, it is mounted within the lithographic system.
A potential advantage of cassettes for loading wafers into lithographic systems is the possibility of machine independence. That is, a variety of commercially available lithographic systems are offered by different manufacturers and in different models. In general, such machines either readily accept wafer cassettes, or are readily modified to accept wafer cassettes. Thus, in principle, a wafer together with a standardized electrostatic chuck could be assembled in any of a variety of different cassettes conforming to different machines, provided only that each cassette accepts the relevant standardized chuck size. In this sense, the chuck would be machine independent.
In practice, however, machine independence has not been achieved for electrostatic chucks. One obstacle to machine independence has been differing power supply requirements between different electrostatic chucks. That is, the various conventional electrostatic chucks that are available have different designs. Some designs require a dc power supply for charging, whereas other designs require an ac power supply for charging. Even designs sharing an ac or dc power requirement generally differ as to the voltage required. Thus, until now no truly machine independent electrostatic chuck has been provided, because any particular electrostatic chuck is likely to be electrically incompatible with all but one or a few models of lithographic system.